1. Field of the Invention
The present invention relates generally to an apparatus"" and method for improving stability in a current feedback circuit and, more particularly, to a method and apparatus for eliminating low-frequency drift in the diode drive circuit of an oximeter.
2. Related Work
A pulse oximeter is a type of blood gas monitor which non-invasively measures an amount of saturation of oxygen in the blood. The saturation of oxygenated blood may be determined from the peak-to-peak voltages or from instantaneous differential voltages for two plethysmographic waveforms measured at separate wavelengths. The two waveforms are produced by driving a visible red light-emitting diode (LED) and an infrared LED to produce two lights that pass through a patient""s tissue, and then detecting the light on an opposite side or same side of the tissue using one or more photodetectors. The light-emitting LEDs are placed in a probe that is attached to the patient""s body in a preferred location for the particular application. Although most conventional oximeters use the red and infra-red LEDs, other devices such as surface emitting laser devices having different wavelengths may also be used, and the number of LEDs can vary according to the specific measurement application. For example, it is known to set a number of laser diodes to be equal to or greater than the number of blood analytes that are to be measured by the instrument.
A conventional photodetector output signal indicates the attenuation of the two different wavelength lights from the LEDs after the lights pass through the patient""s body. In order to obtain a degree of consistency and ease of use, the photodetector is generally placed in a clip or similar device attached to the patient""s finger or earlobe. Attenuation of the lights is substantially constant except for the flow of blood. Thus, the constant attenuation due to the light passing through the patient""s skin and other tissue can be determined and filtered from the photodetector signal, thereby obtaining a signal representing the desired blood oxygen characteristics. Signals containing a component related to a patient""s pulse are known as plethysmographic waves and are used in blood gas saturation measurements. So, for example, the red/infra-red ratio for waveforms at different wavelengths may be analyzed to, obtain oxygenization values.
A photoplethysmographic measurement system used for oximetry typically isolates and separates components of composite detected signals in order to identify and remove noise sources, and then analyzes pulsatile components related to blood oxygen measurements. Conventional systems utilize complex signal processing for filtering noise and separating-out the signals that are due to absorption of the emitted diode light by the patient""s tissue other than the arterial vessels. In oximetry detection and processing, it is critical to reduce noise because signal levels for measuring arterial blood oxygen saturation levels are extremely small. Ambient light can generate high levels of electrical noise unless detectors are shielded and adapted to physically blocking external light during a measurement. Other noise sources are typically present in, for example, a hospital environment. Electromagnetic radiation from various patient monitors may each have their own particular operating frequencies, a hospital may use radiotelemetry in systems such as wireless patient charts, and RF noise from cell phones, computers, and television, etc. may create a combined complex background noise that can combine and interfere with the ambient light noise. Similarly, patient motion causes motion artifacts that create difficulties in providing accurate oximetry measurements.
It is known to activate the red and infra-red LEDs during different time periods, where the two LEDs are cycled on and off alternately, in order to enable the photodetector to receive one signal at a time. As a result of generating LED pulse trains in a time-division manner, a composite time-division signal is then received by the photodetector. Alternatively, switching of LEDs may be related to other parameters such as maintaining a particular duty cycle without regard to time-division multiplexing (TDM). Various methods, not limited to TDM or to periodic switching, for modulating the LEDs can also be employed.
The term xe2x80x9cnoisexe2x80x9d is known to those of ordinary skill in the oximetry art to include any signal portions relating to ambient light, motion artifacts, absorption variance, electromagnetic radiation, electrical interference, magnetic fields, electronic interference such as harmonics or RF, and others. However, conventional diode driver circuits have not been optimized for producing as clean a diode drive signal as possible. There are two general types of noise associated with an op-amp that are relevant to an LED drive circuit in an oximeter. xe2x80x9c1/f noisexe2x80x9d is noise which becomes greater per unit bandwidth as the frequency decreases, whereas xe2x80x9cwhite noisexe2x80x9d remains constant and flat over a broad range of frequencies.
In order to increase the accuracy and resolution of the oximeter, it is desirable to minimize noise in the circuitry used to produce one or more drive currents for causing the LEDs to illuminate. Conventional LED drive circuits have been designed to reduce photic noise generated by the LEDs, in order to maximize a signal-to-noise ratio for the arterial attenuation signal(s) used in processing oximetry data. However, the conventional circuits use ultra low noise operational amplifiers with high gain, which are expensive and inefficient, or simply ignore the problem of a noisy drive circuit because other parts of the oximetry measurement device dominate a system noise. The conventional systems use operational amplifiers that have an ultra low noise specification, which amounts to compensating for rather than eliminating a noise source. As is further discussed below, conventional oximetry systems were not able to see a problem of noise in the diode driver circuit because of conventional signal processing limitations.
Conventional diode drive current circuits may also use xe2x80x9cchopperxe2x80x9d type op-amp configurations that create additional noise when a corresponding LED is on, due to chopping that is not synchronous with the rest of the system.
Other conventional ways to address a noise problem in an oximeter include using filters to reduce the effects of ambient electromagnetic noise in electronic monitoring instruments, especially when the noise source frequency (or a harmonic of the noise source frequency) is approximately the same as the fundamental frequency or harmonics at which the instrument is operating. Another conventional example is a use of a static bandpass filter to remove a portion of the photodetector""s output noise signal that is outside an identified bandwidth of interest, leaving random and/or erratic noise that is within the filter""s passband. A processor has then been used to separate-out primary signal portions in order to isolate and identify the remaining noise signals, which are then removed using, for example, an adaptive noise canceller. Such a scheme is known as correlation canceling.
A method such as correlation canceling used in conventional oximeters simply accounts for various noise sources by identifying and isolating those separate noises in a processing of detected signals. However, such processing does not isolate, reduce, or eliminate a driver circuit noise itself.
Of particular interest for improving the oximeter performance is a problem of low frequency drift (0.5 to 10 Hz) in the LED drive circuitry and in the intensity level of the LEDs, which then creates drift in a detected signal, particularly in low perfusion conditions where the AC (measurement) component of the LED intensity is small. If the power and intensity of the LED varies within the physiologically-related passband (xe2x80x9cphysio passbandxe2x80x9d), the resultant intensity variation becomes an amplified error that is subsequently read as a phisiological signal by a detector, creating an erroneous oximeter reading.
Since a signal-to-noise ratio is increased by decreasing the noise, a decrease in the floor noise allows a lower limit for low-pass filtering within the diode drive circuit. The present invention is addressed to this object as well to generally eliminating low frequency drift in LED intensity. The present invention also has an object of improving detection of low level signals. The present invention also has an object of improving a low frequency noise elimination in an LED drive circuit. The present invention also has an object of improving control of noise generation in an LED drive circuit. For example, the problem of LED intensity variation within the physio passband of oximeter operation can be traced to such noise being generated by the LED drive circuit.
A method according to the present invention includes nulling an operational amplifier (op-amp) disposed in the feedback path of a diode drive circuit.
A method of producing a diode drive current in an oximeter, according to the present invention, can include sensing at least a part of a current passing through the diode and converting the sensed current to a sensed voltage, inputting the sensed voltage to a feedback amplifier for stabilizing the current passing through the diode, and eliminating an offset voltage across inputs of the feedback amplifier.
A pulse oximeter that converts pulses into light flashes, according to the present invention, can include a diode for emitting the light flashes, a feedback amplifier having inputs, a feedback capacitor, and an output, the feedback amplifier being operative to stabilize a current passing through the diode, a nulling amplifier having inputs, a nulling capacitor, and an output, the nulling amplifier being operative to charge and discharge the feedback capacitor until the inputs of the feedback amplifier are at a same voltage.
This summary does not limit the invention, which is instead defined by the appended claims.